Potential to reduce margins and Shrink targets in patients with intact cervical cancer treated on An MRI guided radiation therapy (MRgRT) system.

Introduction: Consensus contouring guidelines for intensity-modulated-radiation-therapy (IMRT) of patients with locally advanced cervix cancer (LACC) advise including the whole uterus in the target volume and adding generous planning-target-volumes (PTVs) to account for motion uncertainties of the gross-tumor-volume (GTV). The primary objective of this analysis was to assess the interfractional GTV motions using a magnetic-resonance-image (MRI) guided-Radiation-Therapy (MRgRT) system to investigate the margins required for MRgRT treatments.

Methods: 125 daily set-up MRIs from five patients with LACC who received MRgRT were analyzed. The GTV, bladder, uterus, and rectum were contoured on all 125 MRIs. Tumor volume changes were calculated in cubic-centimeters (cc). The positional and volume changes of organs-at-risk (OARs) were calculated to assess their effect on GTV interfractional motion, these data were used to calculate adequate PTV margins.

Results: The tumor volume decreased in size during the course of MRgRT for all patients, from 34.0 % to 85.2 %. The interfractional average GTV displacement ranged from 0.46 cm to 0.94 cm. The PTV margins required were: 0.78 cm Left-Right, 1.31 cm Anterior-Posterior and 1.38 cm for the Superior-Inferior directions. The proposed PTV margins, compared to those recommended by consensus guidelines, reduce the PTV by 38 % sparing both the sigmoid and bowel OARs.

Conclusions: By utilizing daily onboard MRI guidance, the GTV becomes readily visualized, allowing for margin reduction and potentially excluding a portion of the uterine fundus from the PTV. The amount of interfractional motion demonstrated in this study is considerable and clinically significant with the goal of decreasing treatment toxicity while maintaining tumor control.

Summary: Daily pretreatment magnetic resonance images (MRIs) from patients with locally advanced cervix cancer (LACC) treated with on-board MR-guided radiation therapy (MRgRT) were analyzed to quantify the range of interfractional motion and develop target volume guidelines for adaptive MRgRT. MRI-guidance leads to better tumor visualization in comparison to cone beam computed tomography (CBCT), and online adaptive planning can account for the interfraction motion of the tumor and surrounding tissue. MRI's ability to better visualize the disease and pelvic anatomy along with adaptive on-board MRgRT could allow for a reduction in the required setup margins as well as potentially excluding non-diseased portions of the uterus from the target volumes. These changes will lead to reduced treatment volumes and may lead to decreased treatment toxicities and allow for dose escalation in certain circumstances.